Image sensing devices made up of an array of laterally spaced sensors are well known and can take a variety of forms. The array can be viewed as being made up of a number of laterally offset regions, commonly referred to as pixels or sensing elements. The art has recognized that sensing advantages can be realized by forming a lens array having a convex lens surface for each pixel. FIG. 1 shows a single pixel 10 on which a photodiode 12 can be seen through a light shield aperture 14 centrally located under a lens 16.
A lens array placed in registration with the device serves to focus the incident light upon the photodiodes. This has the effect of increasing the light gathering area of the photodiode and hence improving the signal.
The device has a lens supporting layer for offsetting the lens from the photodiode to maximize collection of light in the photodiode. The fabrication of lens arrays is based upon using conventional positive-working photoresists. A coating of the photoresist is exposed and developed to produce an array of resist islands. The resist pattern is then flood exposed to bleach the remaining photochemistry and subsequently heated until the resist material flows enough to form a lens and yet not so much as to cause adjacent lenses to flow together. Therefore, there has to be some spacing between the lenses.
The function of the photodiode is well known and produces electrons in proportion to the amount of light received on exposure. The problem with this arrangement is that light is focused onto a central region 17 of the photodiode. Since the light shield aperture and photodiode are elongated and the lens base is essentially square to match the pixel shape, the lens must be designed to focus the light through the smallest side of the light shield aperture. Therefore, only a portion of the photodiode is used to receive light. The efficiency of this type of arrangement is therefore not maximized.